The term “flame resistant” is used to describe a material that burns slowly or that is self-extinguishing after removal of an external source of ignition. A fabric or yarn may be flame resistant because of the innate properties of the fiber, the twist level of the yarn, the fabric construction, or, as will be discussed herein, the presence of flame retardant chemicals applied to the fabric.
The term “flame retardant” or “flame retardant chemical” refers to a chemical compound that may be applied as a topical treatment to a fiber, fabric, or other textile item during processing to reduce its flammability. In the present case, flame retardant chemicals are applied to the already constructed fabric substrate to produce a flame resistant fabric.
Flame resistant fabrics are useful in many applications, including the production of garments worn by workers in a variety of industries, including the military, electrical (for arc protection), petroleum chemical manufacturing, and emergency response fields. Cellulosic or cellulosic-blend fabrics have typically been preferred for these garments, due to the relative ease with which these fabrics may be made flame resistant and the relative comfort of such fabrics to the wearer.
Conventionally, to achieve such flame resistant properties in cellulosic-containing fabrics, the fabrics are subjected to an “ammonia process” or “ammoniation process” in which the target fabric is dipped in a bath containing a phosphorous-based flame retardant chemical, dried at relatively low temperatures, conveyed through a chamber containing gaseous ammonia, and then dipped in separate baths of peroxide and caustic before drying.
The first step of the ammoniation process involves reacting a tetra(hydroxymethyl)phosphonium compound with urea to produce a THP pre-condensate. (Such pre-condensates are commercially available, under tradenames such as PYROSAN® CFR from Emerald Performance Materials, and, accordingly, the synthesis of these compounds is omitted from the illustration below.) As shown below, the pre-condensate is reacted on the fabric surface with gaseous ammonia (typically with 15% moisture) to create an intermediate compound in which the phosphorous compound is present in its trivalent form.

To fix the flame retardant compound to the fabric surface and to convert the trivalent phosphorous to its stable pentavalent form, the treated fabric is conveyed through a peroxide bath, in which the peroxide oxidizes the phosphorous compound. This step is illustrated below.

Ammoniated cellulosic fabrics have relatively good flame retardance, particularly in those instances in which cellulosic fibers comprise the majority of the fiber content. Another advantage of such ammonia-treated fabrics is that they tend to exhibit a soft hand and good tear strength.
However, there are several drawbacks that have been identified with the ammoniation process. One obvious disadvantage of this process is the high capital investment associated with installing an ammonia chamber and requisite environmental controls, as well as the expenses associated with its operation and maintenance. Another processing disadvantage is the limitation on the application of other finishing agents (e.g., soil repel agents, stain release agents, permanent press resins, and the like), because these finishing agents require high temperatures for fixation, which would result in the generation of malodors on the treated fabric.
Moreover, in terms of fabric properties, the flame resistant properties produced by this process tend to lack wash durability due to the amine (—NH) linking groups between the molecules and the method of setting the flame retardant chemistry on the fabric. Besides lacking durability to repeated launderings, these fabrics often have poor wrinkle resistance and appearance retention. Also, because the flame retardant chemistry interacts primarily with the cellulosic fibers, the maximum amount of synthetic fiber that may be used is on the order of about 20-30% by weight.
For fabrics having a high cellulosic content (including those made entirely of cellulosic fibers), the present chemistry and process provide a cost-effective solution to the problems outlined above. Such fabrics possess a flame retardant treatment that is durable for multiple industrial launderings, a softness suitable for apparel applications, and an above-average tear strength for flame retardant cellulosic fabrics. The present treated fabrics also exhibit good appearance retention, achieving results similar to those achieved by the separate introduction of permanent press resins. Optionally, additional finishing agents (such as soil repel agents) may be applied to the fabrics, either simultaneously with the flame retardant chemistry or after the application of the flame retardant chemistry, to impart desirable properties without concern over generation of malodors at the temperatures required to heat-set such finishing agents. For these reasons, the present chemistry and process represent an advance over the ammoniation process.
It is well-known that fabrics with such high cellulosic content tend to exhibit deficiencies in terms of durability, abrasion resistance, and drying time. Where these shortcomings pose a serious detriment, manufacturers have tried, with varying degrees of success, to incorporate higher percentages of synthetic fibers into these fabrics. The difficulty with accommodating the desire for more durable substrates are the tendency of the synthetic fibers to burn or melt and the tendency of the (hydrophobic) synthetic fibers to resist penetration of the flame retardant, thereby making them unsuitable for use in large percentages. Thus, when using the ammoniation process to impart flame resistance to fabrics having a blend of cellulosic and synthetic fibers, the amount of synthetic fiber content has heretofore been limited to less than 30%. As mentioned briefly above, the ammonia process tends to preferentially bind the flame retardant chemical to the cellulosic fibers in the fabric.
The present process overcomes the shortcomings of the previous approaches—regardless of the amount of synthetic content in the fabric—by providing an alternative mechanism by which one or more flame retardant chemicals may be fixed on a target textile substrate. As a result, the fabrics exhibit a durable finish, and larger amounts of synthetic fibers may successfully be incorporated into the fabrics without a loss of flame resistance. These larger amounts of synthetic fibers contribute significantly to increasing the durability and the tear strength of the treated fabrics. Even in fabrics having a low synthetic content, the present process imparts flame retardant properties in an economically advantageous way, while overcoming the shortcomings associated with the ammoniation process used previously.